Detecting new user signup in [...nextauth].js

I'm building an app with NextAuth's email provider as the sole means of user registration and login.

In local development everything worked fine, but when deploying the app to Vercel I keep getting Unhandled Promise Rejection errors from Mongo, so I decided to switch to NextAuth's unstable_getServerSession and everything is running very smoothly now.

Before making the switch I used an extra parameter to communicate whether the user was just logging in or newly signing up. Based on this, I sent different emails (a welcome email with login link vs. a shorter message with just the login link).

Here is how the [...nextauth].js file looked before:

export default async function auth(req, res) {

    const signup = req.body.signup || false

    return await NextAuth(req, res, {
        adapter: MongoDBAdapter(clientPromise),
        ...
        providers: [
            EmailProvider({
            ...
          sendVerificationRequest(server) {
            customLogin(
                server.identifier,
                server.url,
                server.provider, // passing the above data for email content
                signup // custom parameter to decide which email to send
            )
          }
          })
        ],
        ...
    })
}

I adjusted the code sample from the documentation, but wasn't able to pass a custom parameter to it.

Here is my new code:

export const authOptions = {
    adapter: MongoDBAdapter(promise),
    ...
    providers: [
        EmailProvider({
        ...
      sendVerificationRequest(server) {
        customLogin(
            server.identifier,
            server.url,
            server.provider,
        )
      }
      })
    ],
    ...
}


export default NextAuth(authOptions)

The customLogin function doesn't do anything except construction the different email options.

Among the things I have tried are wrapping authOptions in a handler function with req and res, setting up a separate API route, and passing parameters to the NextAuth function, but none of those worked.

Any advise on how to implement a custom parameter here would be highly appreciated.